Probe storage memories (also known as Seek and Scan Probe or SSP memories) are a type of chip scale mass storage devices in which read/write access to the memory cells is achieved micro-mechanically by moving read/write heads over the storage medium. The memory cells in the storage medium are not lithographically defined as in conventional semiconductor memories, but rather are created or read as sea of bits with a precision set by the accuracy and resolution of the micro-mechanical system that positions the read-write heads. Parallelism in the form of multiple read write heads is often used to combat the micro-scale distance traveled by the precision micro-mechanical movers. In addition since high capacity is needed in chip scale formats, the arial density of the storage media also needs to be high and is typically >2 Tbits/in2. Compared to semiconductor memories the cost/bit of probe storage can be lower as it does not require advanced lithography and the micromechanical access system can be fabricated on older semiconductor technologies. Typical probe based memories include storage media made of materials that can be electrically switched between two or more states having different electrical characteristics such as resistance, polarization dipole direction, or some other characteristic.
SSP memories are written to by passing an electric current through the storage media or applying an electric field to the storage media. Passing a current through the storage media, or applying an electric field to the media, is typically accomplished by applying a voltage between a sharp probe tip on one side of the storage media and an electrode on the other side of the storage media. Current SSP memories use probe tips positioned on a free end of one or more MEMS probes. In an idle state each MEMS probe maintains the probe tip at a certain distance from the storage media, but before the electric field or current can be applied to the storage media the probe tip must usually be brought close to, or in some cases in direct contact with, the storage media.
FIG. 1 illustrates the basic concept of a probe storage device and compares a probe storage device with a semiconductor memory. On the left is a conventional memory with memory cells addressable by word lines and bit lines. On the right is a SPP memory where bits are stored on a media on a moving sled read by probes. FIGS. 2A and 2B illustrate an SSP memory configuration in which an array of cantilever probes are anchored to a substrate (the cantilever wafer), and can be actuated to contact or de-contact the storage media on a mover that carries a storage media and is positioned over the cantilever wafer.
FIGS. 3A and 3B illustrate how data bits are organized in the storage medium the form of tracks. The data tracks are stored in the storage media in one of two ways, depending on how the media mover scans relative to the cantilever tips. FIG. 3A illustrates axial scanning, where data is stored in the storage media in-line with the cantilever direction, such that the mover scans in the direction parallel to a longitudinal axis of the cantilever to read/write/erase (R/W/E) each data track. FIG. 3B illustrates transverse scanning, where the media mover scans in a direction perpendicular to the longitudinal axis of the cantilever probe to R/W/E each data track; data is consequently stored in lines that are transverse to the cantilever's longitudinal axis.